In order to show the usefulness of Cypridina luciferin analogues, 2-methyl-6-phenyl- and 2-methyl-6-(p-methoxyphenyl)-3,7-dihydroimidazo[1,2-a]pyrazin-3-ones for the specific detection of active oxygen species, chemiluminescence rates, and intensities, including their quantum efficiencies were measured and determined by using various oxidants in aqueous conditions. The oxidant-specificity of their chemiluminescence to both singlet molecular oxygen and the superoxide anion radical was shown experimentally for the first time.